The Roaring Forties is the term for a band of prevailing strong westerly winds that blow between latitudes 40 and 50 degrees south. In the age of sail they were used by mariners on westward voyages across the southern Indian Ocean first, from 1610, by the Dutchman Hendrick Brouwer on his way to the East Indies and, from the late 18th century, by the British to Australia and later to New Zealand.
Ships would sail south from Cape of Good Hope searching for these winds before which they could then "run free" under full sail, often making particularly fast times. (“The Oxford Encyclopedia of Maritime History”, in its article on the Indian Ocean has stated that vessels could reach 30 knots in the right conditions.) By the mid-nineteenth century these ships, usually the big full-rigged ships or clippers, were said to "run the easting down".
Two straits are affected by the Roaring Forties. Bass Strait between Tasmania and the mainland of Australia and Cook Strait between the two main islands of New Zealand are known for the fierceness of their gales, especially around the time of the spring equinox – September. The north-westernmost point of land in Tasmania where Bass Strait becomes the Indian Ocean is named Cape Grim.
While the term was created to refer to the Indian Ocean passage it is now also used for other ocean parts that are within latitudes 40 to 50, both north and south, where gales are known to occur. Before the Panama Canal was built, ships were forced to go even further south to round Cape Horn at the bottom of South America.
In the last fifty years, weather patterns have shifted poleward. So the "Roaring Forties" have been replaced by the "Furious Fifties." Westerly winds have quickened 10 to 15 percent, and the weather system has moved 2 to 5 degrees southward.
- "Wilder winds, less rain, as Roaring Forties become Furious Fifties", Sydney Morning Herald, May 11, 2014.